The present invention relates to a recorder and player employing semiconductor memory as a medium on which an audio signal is recorded.
At present, semiconductor memory is used as a recording medium for voice signals in devices such as telephone answering machines and automobile navigation systems. The voice signals are digitized and compressed by a method such as adaptive differential pulse-code modulation (ADPCM), linear predictive coding (LPC), or code-excited linear predictive coding (CELP). These methods are substantially limited to voice signals, because they lack the frequency range and dynamic range needed for recording music signals with satisfactory quality.
Music and other quality-sensitive audio signals are generally recorded on magnetic tape or optical discs. Compact, inexpensive recorders and players employing these media are commercially available, and the compact magnetic tape cassette and compact optical disc (CD) have become standard audio media worldwide, the former for both recording and playback, the latter for playback only.
These media have a considerable cost advantage over semiconductor memory. A single CD, for example, can hold a one-hour stereo recording comprising over six hundred megabytes of audio data. Semiconductor memory with an equivalent capacity would be prohibitively expensive at present. Moreover, the quality of an audio signal reproduced from a CD is excellent.
More advanced tape and disc media have also been developed and commercialized. An early example was the digital audio tape (DAT), which enabled audio signals to be recorded on magnetic tape with CD-level quality. A more recent example is the digital compact cassette (DCC), a magnetic-tape medium developed by the Philips Corporation. Another example is the minidisc (MD) developed by the Sony Corporation, a type of magneto-optical disc which is smaller than a CD, and permits recording and erasing as well as playback.
Tape and disc media are driven by motors, however, and scanned by a magnetic or optical head. Even the most compact tape and disc recorders and players must therefore be large enough to accommodate the motor and head. If battery-driven, the motor consumes power at a rate that makes frequent battery recharging or replacement necessary. Tape and disc recorders and players moreover have moving parts that tend to wear out, and disc recorders and players, in particular, are sensitive to mechanical shock. In minidisc devices, this necessitates a large buffer memory circuit, referred to as a shock-proof memory. Overall, tape and disc devices leave much to be desired in terms of size, weight, battery life, ruggedness, and durability.
A further problem is that audio signals are recorded on tape and disc media in a variety of incompatible formats, using different encoding and compression methods. A brief summary follows.
A compact cassette stores audio signals on magnetic tape in analog form, without compression.
A CD stores audio signals in pulse-coded modulation (PCM) form, without compression, but with cross-interleave Reed-Solomon coding (CIRC) for error correction, and eight-fourteen-modulation channel encoding for accurate track-following by the optical pick-up head. The PCM sampling frequency is 44.1 kilohertz (44.1 kHz), with sixteen bits per sample.
A digital audio tape (DAT) also stores audio signals in PCM form, with a Reed-Solomon error correction code. The sampling frequency is 32 kHz, 44.1 kHz, or 48 kHz, with twelve or sixteen bits per sample.
A minidisc (MD) stores audio signals in a digital form compressed by adaptive transform acoustic coding (ATRAC), with eight-fourteen-modulation channel encoding. The sampling frequency is 44.1 kHz. ATRAC is a perceptual coding method that applies a Fourier transform to blocks of audio samples, converting each block to a frequency spectrum, then compresses each converted block according to psycho-acoustic principles. This type of compression enables the audio data to be stored at about three bits per sample without perceptible loss of quality. Since disc recording is apt to produce burst errors, an added-on cross-interleave Reed-Solomon error correction code is used.
A digital compact cassette (DCC) stores audio signals in a digital form compressed by precision adaptive subband coding (PASC). PASC resembles ATRAC in using psycho-acoustic principles, but differs in using subband analysis instead of the Fourier transform, and also differs in the compressed data rate and format. The error correction code is a C.sub.1 -C.sub.2 Reed-Solomon code, and eight-ten modulation is adopted, for maximum recording and playback efficiency.
The Motion Picture Experts Group (MPEG) has standardized several methods of encoding digital audio signals. The different methods are identified by different layer numbers. All layers offer a selection of compression ratios; that is, the output bit rate can be varied. PASC is a version of MPEG Layer One. The higher MPEG layers are increasingly complex, but offer increasingly better sound quality for a given bit rate. The MPEG standards are currently used for audio signal transmission and broadcasting, as well as for audio recording.
In all of the above digital recording methods, the digital audio signal is accompanied by auxiliary information, referred to as a subcode or as digital management data, for example. The format and content of this auxiliary information varies from method to method.
Conventional tape and disc recorders and players are generally limited to the use of just one of these different standard coding methods and formats. As a result, many recorders and players are incompatible with many media. A further problem is that conventional audio recorders and players cannot be easily interfaced with a computer, either because of an incompatible input/output interface, or because of incompatible data formats.